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Wetlands of the Bitterroot Valley : change and ecological functions

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Wetlands of the Bitterroot Valley : change and ecological functions


Published 2008
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"January 2008"-cover date

Literature cited: p. 29-32

The objective of this project was to evaluate wetland diversity and analyze wetland change in the Bitterroot River watershed of western Montana. This watershed is typical of many large river valleys in the West that have a rapidly increasing affluent population expanding into rural areas. We characterized the spectrum of wetland types and analyzed wetland change by comparing the original National Wetland Inventory (NWI) baseline in the early 1980s to our new NWI mapping based on year 2005 imagery. We also developed a system for labeling mapped wetland polygons using a combined NWI - HGM (Hydrogeomorphic) code. Codes have been associated with three performance levels of 10 ecological functions. This enabled us to represent hydrology, biogeochemical, and habitat wetland functions on wetland maps, and represents the most detailed mapping information system for wetlands that has ever been implemented on a statewide or regional basis. This information will help decision-makers prioritize wetlands for restoration or protection, guide mitigation requirements, support regional or local wetland policy and management, and will contribute to a broader understanding of the wetland ecological services that society values. We found that wetlands and wetland associated ecological functions are concentrated in the valley bottom and along riparian areas. Most of the 442 Clean Water Section 404 Program permits that have been issued within the study area are concentrated in the wetland-rich riparian floodplain. Some of the permitted activities, like armoring banks with rip-rap, may limit the ability of the river to maintain the same amount of wetlands on the floodplain because high flow events are essential in creating and renewing wetlands. Wetland change was analyzed in two ways. A random sampling indicated no net estimated change in total wetland acreage, using confidence limits that were relatively large due to high sampling variability. However, we did find that ponds increased in estimated acreage, whereas the estimated acreage of emergent wetlands, which were often converted into ponds, decreased. We additionally completed a total study area review of ponds created by humans and beavers, and found an 80% decrease in beaver pond numbers and acreage during the approximately 20 year study time frame. Only about 5 acres of beaver ponds remain in this 1.4 million acre area despite the large amount of suitable beaver habitat. Beavers are a keystone species with a disproportionate effect on ecological functions compared to their numbers. Beaver activity improves water quality through sediment retention, influences on nutrient cycling and decomposition, and hydrologic modifications. Beavers create wetlands that would otherwise be rare in mountainous terrain, thus providing important habitat for many other wetland-dependent species. The other major wetland change in our 100% review was a 75% increase in human created Palustrine wetland acreage. The 921 new created wetlands in the study area since the early 1980s are virtually all small ponds with standing water that were primarily constructed for their recreational amenities. Fish stocking is a major use, 252 fish stocking permits were recorded since they were first required in 1998. Over 90% of the permits indicated an intention to stock non-native fish species. The presence of fish in a pond has also been strongly and negatively associated with the populations of some amphibian species in Montana. Only about 30% of these ponds had the required water use permit for pond construction. We estimated ecological functions for created ponds as generally lower than natural ponds, but there is considerable uncertainty about actual functional levels due to a lack of research and potentially large ecological impacts associated with the spread of the non-native bullfrog, a problem species in the area, and a general decline of native amphibians across Montana. If constructed wetlands do not function like natural wetlands, then landscape wetland functions may still be lost even with a gain in wetland acreage. Wetlands and deepwater types comprise 1.1% (16,304 acres) of the total study area. Over 1,806 acres, 11% of the total wetland acreage, are isolated wetlands, which may not be regulated. The flooded beds and shores of rivers are the most common wetland type (34%), followed by wetlands with emergent vegetation (26%), deepwater habitats (13%), and wetlands with shrub vegetation (12%). Forested wetlands are very uncommon, only 15.1 acres were mapped. The location of most wetlands in riparian corridors and on the valley floor near human developments has often resulted in a degraded ecological condition with many emergent wetlands converted to pastures with introduced grasses. Nonnative species and noxious weeds are common, especially in the riparian zone of the Bitterroot River. Higher elevation wetlands are more ecologically intact; wetlands with a saturated water regime are more common there than in most of Montana. These types are often peatlands that may provide habitat for Montana plant Species of Concern and the Northern Bog Lemming, an animal Species of Concern. About 38 acres of slope wetlands were mapped; these also have the potential for high conservation value. Our new map data provides a valuable tool for field botanists to explore these areas. The considerable change in wetlands of the Bitterroot Valley after only 20 years underscores concern about a changing profile of wetland values and services. More effort is needed to understand the impacts of large increases in created recreational ponds. The large decrease in beaver ponds, and presumably beaver numbers, is also worthy of additional focus. Beavers and humans often share riparian areas and it is likely that beavers are diminishing due to this relationship

Agreement Number


Volume 2008
Publisher Helena, Mont. : Montana Natural Heritage Program
Pages 80
Language English
Call number 577.68
Book contributor Montana State Library
Contributor usage rights See terms
Collection MontanaStateLibrary; americana

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